Demodulation systems for the chrominance subcarrier of a video signal require use of bandpass filters accepting a signal with a relatively large dynamic range. The bandpass filters have quality coefficients defining the selectivity of these filters. These quality coefficients are relatively high, e.g., within the range 3 to 30.
In prior art demodulator circuits, these filters can be produced using integrated capacitors and transconductors. A transconductor is defined by its transconductance g.sub.m such that its output current i.sub.s =g.sub.m *.upsilon..sub.e. The variable .upsilon..sub.e is the voltage between the two input terminals. These transconductors must be able to be regulated to adjust the center frequency of the filter. The control of transconductance is provided by a servoloop using a filter structure analogous to that of the filter to be adjusted and a precise frequency reference.
A traditional example of a bandpass filter is illustrated in FIG. 1, and is described in an article titled "Active Filter Design Using Operational Transconductance Amplifiers: A Tutorial," by Randall L. Geiger and Edgar Sanchez-Sinencio, published in IEEE Circuits and Devices Magazine, March 1985, pages 20-32. More particularly, a description of this bandpass filter can be found in this reference on the final two lines on page 25, and the first fifteen lines on page 27 viewed with FIG. 7(a) therein. This reference discloses various structures for circuits (e.g., voltage controlled amplifiers, filters, etc.) based on transconductors for integration.
The filter illustrated in FIG. 1 includes first and second transconductors 10 and 11, respectively with transconductances g.sub.m1 and g.sub.m2. The first input (+) of the first transconductor 10 is connected to ground. The second input (-input or reverse input) of the first transconductor 10 is connected to the second input (-input or reverse input) of the second transconductor 11, and to the output S of the filter. The output of the first transconductor 10 is connected to the first input (+) of the second transconductor 11, and to the input E of the filter through a first capacitor C1. The output of the second transconductor 11 is connected to ground through a second capacitor C2, and to a monitor amplifier 12. The output of the monitor amplifier 12 is connected to the output S of the filter.
This filter can be formed, for monolithic applications, in MOS or bipolar technologies. The transfer function of this filter is such that: ##EQU1##
The variable p is the Laplace variable with the resonant frequency defined as follows: ##EQU2##
The variable .omega..sub.0 can be adjusted as a result of regulating g.sub.m1 and g.sub.m2.
The quality coefficient is such that: ##EQU3##
By choosing the transconductances g.sub.m1 and g.sub.m2 so that the ratio g.sub.m1 /g.sub.m2 remains constant, a quality coefficient value Q that is independent of .omega..sub.0 is obtained. To provide a precise adjustment of .omega..sub.0, it is then necessary to vary the transconductances g.sub.m1 and g.sub.m2 in the same proportions, and to ensure the best possible matching between the transconductances g.sub.m of this filter and the transconductances g.sub.m of the servo-loop filter.
Firstly, this leads to a limiting of the ratio g.sub.m1 /g.sub.m2 to a value that does not exceed 4. The value of the quality coefficient can then be fixed by choosing the ratio C.sub.2 /C.sub.1. In order to obtain a high quality coefficient, the ratio C.sub.2 /C.sub.1 must be large. This is incompatible with the constraints of circuit integration. In effect, poor matching of C.sub.1 and C.sub.2 is obtained if C.sub.2 &gt;&gt;C.sub.1. The parasitic capacitances which are the same size as C.sub.1 cause inaccuracy in the filter.
The output node 13 from the first transconductor 10 is particularly critical. In effect, it is at a high impedance. Small parasitic capacitances can cause changes in filter characteristics. Furthermore, a large overvoltage linked to the quality factor develops at this node 13.
At resonance .omega.=.omega..sub.0, the voltage .upsilon..sub.c1 this node 13 is of the form: ##EQU4##
The input voltage dynamic range .upsilon..sub.e is then Q times less than the output dynamic range of the first transconductor 10. Such a limitation means that a high penalty is paid in circuits with a low supply voltage.